In an attempt to provide an improved assessment of muscle inflammation in patients with myositis, we have evaluated MRI. In a group of myositis patients, muscle biopsy, laboratory tests, and the "STIR" image of the thighs were compared to an assessment of clinical disease activity. MRI was more sensitive than biopsy in detecting inflammation in these patients. The extent of muscle inflammatory changes could be assessed in patients since the entire muscle group is imaged. Magnetic resonance spectroscopy allows measurement of the metabolic state of muscle cells. Profound abnormalities of inorganic phosphate/creatine phosphate have been found in inclusion body myositis and in the tryptophan-induced eosinophilia-myalgia syndrome, and lesser abnormalities in polymyositis and dermatomyositis.